You did not mentioend what sound card you have. Did you use the soundcards drivers and installed software to speciify your speaker configuration.
With a SoundBlaster you have to use the AudioHq to specifiy what sort of environment you have and speaker configuration.
If that does not work or is not supported then you might want to look for an updated driver for the soundcard you have.
And also, this might not work correctly with Winamp, since you state the problem changed while using different versions of winamp. you might just have to use winamp5 or something else.
try a different program for playing your audio, see if the speakers all work, if they do, then problem is with the winamp you have.